In a general continuous molten metal plating process, gas wiping is performed as shown in FIG. 6. In the gas wiping, a gas is jetted from a gas wiping nozzles 21 opposing each other onto the surface of a steel strip X between the gas wiping nozzles 21 that has been immersed in a plating bath 20 containing a molten metal and then drawn up in the vertical direction from the plating bath 20. In FIG. 6, reference numeral 22 designates a sink roll, and reference numerals 23 and 24 designate support rolls. The gas wiping scrapes and removed the excess of the molten metal to control the amount of plating, and uniformizes the molten metal deposited on the surface of the steel strip in the width and the length direction. The gas wiping nozzle generally has a larger width than the width of the steel strip so as to cover the widths of various steel strips and the displacement in the width direction of the drawn steel strip, thus extending over the ends of the steel strip in the width direction.
In such gas wiping, the gas jet is disturbed by collision with the steel strip and causes splashes. The molten metal dropping below the steel strip splashes around. The splashes are attached onto the surface of the steel strip and degrade the quality of the surface of the plated steel strip.
In order to increase the production in a continuous steel strip process, the line speed of the steel strip can be increased. However, the increase in line speed increases the initial amount of the plating on the steel strip immediately after dipping the steel strip in the plating bath because of the viscosity of the molten metal. For controlling the amount of plating in a predetermined range by gas wiping in a continuous molten metal plating process, accordingly, the pressure of the gas jetted onto the surface of the steel strip from the gas wiping nozzles must be increased. This significantly increases splashes to impair the superior quality of the surface.
Accordingly, some methods are proposed to solve the problem. The methods use auxiliary nozzles (secondary nozzles) additionally provided above and below the gas wiping nozzle (primary nozzle) that mainly controls the amount of the molten metal deposited on the steel strip so that the secondary nozzles enhance the performance of the primary nozzle.
Patent Document 1 discloses a method that partially enhances the gas wiping performance in the width direction by providing auxiliary nozzles at the upper sides of the ends of the wiping nozzles to prevent edge overcoating, and by aligning the positions of the steel strip that are hit by jet gas from the auxiliary nozzles and jet gas from the wiping nozzle.
Patent Document 2 discloses a method that prevents the gas jet from a primary nozzle from diverging by jetting a gas from auxiliary nozzles (secondary nozzles) provided above and below the primary nozzle and capable of controlling the pressure independently for regions divided into at least three. The method thus stabilizes the gas flowing along the steel strip after hitting the steel strip.
Patent Document 3 discloses a method in which the primary nozzle and the secondary nozzle are divided by a partition plate whose end at the jetting port side has an acute angle, and the secondary nozzle is tilted 5° to 20° from the primary nozzle to increase the potential core. Thus, the controlability of the plating amount is enhanced to stabilize the gas jet, and consequently noises are reduced.
Patent Document 4 discloses a method in which the primary gas jet is isolated from the ambient air by use of flame as an isolation gas when the primary gas is jetted. By surrounding the primary gas jet by a high-temperature gas, the flow resistance of the primary gas jet is reduced. Consequently, the potential core is increased to enhance the hitting force.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 63-153254    Patent Document 2: Japanese Unexamined Patent Application Publication No. 1-230758    Patent Document 3: Japanese Unexamined Patent Application Publication 10-204599    Patent Document 4: Japanese Unexamined Patent Application Publication 2002-348650